Neuroglial cells surround nerve fibers and play important roles in the physiology and pathology of the nervous system. In unmyelinated neurons, apposing glial membranes reduce the immediate extracellular space to a layer just l00-300 angstrom thick. Electrical activity results in changes in the ionic composition of the perineural space and these alterations, in turn, may modulate further activity. The mechanisms by which neurons and glia regulate the composition of this region are not well understood. We plan experiments to test a number of hypotheses regarding the control of potassium ion concentration in the perineural space. Using the voltage-clamped, internally perfused axon, we shall investigate possible specialized diffusion pathways in glia, passive mechanisms of uptake in neurons, and relative roles of neurons and glia in active transport. In myelinated fibers glial membranes act to improve cable properties of neurons and may also control the distribtuion of ionic channels in the axon membrane. The pathophysiology of multiple sclerosis and other demyelinating diseases is, however, not well understood. Conduction can persist or recur in regions of extensive demyelination. We propose to study conduction in demyelinated segments using optical techniques and potential-sensitive dyes. These measurements should provide a map of membrane ionic current along the fiber and allow a determination of the corresponding distribution of sodium channels. We are aiming at a resolution adequate to examine relatively small regions of paranodal demyelination. We wish to determine if the distribution of sodium channels changes at different states of demyelination. In remyelinating fibers new nodes of Ranvier are formed in previously myelinated axon regions. We shall attempt to characterize ionic channels at these nodes by voltage clamp techniques. In a somewhat different project we are studying ionic channels in neurons of animals that synthesize tetrodotoxin, a molecule with a very high affinity for block of sodium channels. Further knowledge of the relationship between the receptor for tetrodotoxin and functional regions of the sodium channel should be useful in biochemical characterization and isolation of membrane components.